The Lifestyles Intervention for Seniors with Arthritis (LISA) Trial is a single-blind, controlled clinical trial to test the efficacy of weight loss on disability and disease progression in older people with knee osteoarthritis. Three hundred obese, sedentary people, age sixty and older, with symptomatic knee osteoarthritis will be randomized to receive: usual care, a structured aerobic exercise intervention, a dietary weight loss intervention or a combined exercise-weight loss intervention. Participants in the study will be followed for thirty months. The primary outcome measure is physical disability. Secondary outcome measures include performance measures of function, exercise capacity, health-related quality of life, biomechanics of gait, knee pain and disease progression by x-ray. The cost effectiveness of the interventions will be determined. This study will determine the relative efficacies of exercise, weight loss and combined intervention in the management of osteoarthritis of the knee in older people.